all: Become a Go module (fixes #5148) (#5384)

* go mod init; rm -rf vendor

* tweak proto files and generation

* go mod vendor

* clean up build.go

* protobuf literals in tests

* downgrade gogo/protobuf

vendor/github.com/jackpal/gateway/README.md

@@ -0,0 +1,18 @@
+# gateway
+
+A simple library for discovering the IP address of the default gateway.
+
+
+[![Build Status](https://travis-ci.org/jackpal/gateway.svg)](https://travis-ci.org/jackpal/gateway)
+
+Provides implementations for:
+
++ FreeBSD
++ Linux
++ OS X (Darwin)
++ Solaris
++ Windows
+
+Other platforms use an implementation that always returns an error.
+
+Pull requests for other OSs happily considered!